This is not a functional change.

Also:
- minor cleanups, better comments
- uniform spelling of noun "zeros" (per OED)

Fixes #11277.

Change-Id: I1726f358ce15907bd2410f646b02cf8b11b919cd
Reviewed-on: https://go-review.googlesource.com/11267Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>